GANOIDS. 251 



appendages. The dorsal and anal fins are posterior in position, the anal 

 being anterior to the dorsal. In Acipenser there is a spiracle, and the 

 naked skin between the plates extends to the tail. About twenty living 

 species are known, half a dozen from North America. From the ovaries 

 caviare is made, while the air-bladders furnish isinglass. The genus 

 appear in the London clay (eocene). The shovel-nose sturgeons {Scaphi- 

 rhynchus'), one species of which is American, lack the spiracle, have the 

 plates forming a complete armor on the depressed tail, while the caudal fin 

 ends in a filament. In the Polyodontid-e (Selachostomi), represented 

 to-day by Polyodoii spathula in the U.S., and Psephurus in China, the skin 

 is smooth, the snout is prolonged into a long blade (whence the name paddle- 

 fish), the maxillary is obsolete, a spiracle (lacking pseudobranchs) occurs. 

 The PaL/EON'ISCID^, which range from the Devonian to the lias, have 

 small conical or styliform teeth, simple dorsal and heterocercal tail, and 

 rhombic scales. PalcEOniscus, Europe, U. S. ; Eurylepis, U. S. Allied 

 is Platysomus from the carboniferous of Europe and Illinois.i 



ORDER III. HOLOSTEI. 



Ganoids with well-ossified skeletons ; tail heterocercal ; body 

 with ganoid or cycloid scales ; fulcra frequently present ; branch- 

 iostegals and operculum well developed, and frequently a median 

 gular plate ; mouth terminal, with teeth ; fins without a scaled 

 basal region ; the ventrals with the proximal skeletal elements 

 reduced, much as in teleosts. 



The garpikes, Lepidosteid^e (Ginglvmodi) are closely related, struc- 

 turally, to the palasoniscid forms of the chondrostei. They have opistho- 

 ccelous vertebrae, the body covered with rhombic scales, greatly elongate 



Fig. 253. Garpike, Lepidosteus osseus, from Tenney. 



jaws, these, like the vomers and palatines, with sharp teeth ; and numerous 

 pyloric caeca. The living species of the only existing genus Lepidosteus 

 are all American. The common garpike, L. osseus, is widely distributed ; 



1 The group Acanthodid.e, which combines ganoid and elasmobranch characters, 

 may be mentioned here. The cartilaginous skeleton, spine in front of the dorsal, absence of 

 opercular elements, are elasmobranch characters, while the presence of spines in the pectorals, 

 and especially of bones in the orbital region and in the roof of the cranium, and the absence 

 of claspers, recall the teleostcmes. These forms occur in palaeozoic rocks. Acanthodes occurs 

 in U. S. 



